1. Field of the Invention
The invention relates to a drive unit, in particular for an injection unit or an ejector of an injection molding machine.
2. Description of Related Art
Such a drive unit is disclosed in U.S. Pat. No. 5,980,235. FIG. 4 shows a drive unit for an ejector, comprising a stroke spindle device with a spindle nut and a spindle, by which an ejector plate is adapted to be moved axially. The spindle nut is arranged rotatably in a base plate. In support of the stroke spindle device during retraction of the ejector plate, springs are positioned between the stroke spindle device and the ejector plate, said springs being tensioned when the ejector plate moves forward and correspondingly relieved when it retracts.
The disadvantage of this solution is that the springs have a retraction force that depends on the distance. It is further disadvantageous that the springs are subject to symptoms of fatigue, so that readjustment becomes necessary if they are used for a lengthy time. Moreover, the force transmittable for regulating the injection pressure is difficult to collect since, on the one hand, the force applied by the stroke spindle device has to be collected and, on the other hand, the force applied by the springs has to be collected.
WO 02/085599 A1 shows a drive unit for an ejector in which an injection-molded component is to be ejected by means of impact momentum. A stroke spindle device forms, together with an ejector plate, a swinging system that is positioned to swing in axial direction between a first and a second spring package. The first spring package is prestressed and relieved on loosening of an arrest, so that it acts on the opposite second spring package, whereupon the entire spring system is made to swing. For offsetting friction losses, the stroke spindle device is driven by a motor in correspondence with the oscillation frequency of the swinging system.
The disadvantage of this solution is that the spring forces practically cancel each other out and do not or just unsubstantially support the motor driving force of the stroke spindle device. The stroke spindle device rather only acts as an offset for the losses of motion of the spring packages. Furthermore, the motor triggering of the stroke spindle device in correspondence with the oscillation frequency requires expensive controlling. Another disadvantage is that with this solution, too, the injection force characteristics are hard to collect.